Phosphazenes and their preparation are disclosed in U.S. Pat. No. 2,109,491. Partially substitued materials are mentioned in U.S. Pat. No. 3,206,494. p-Bromophenoxyphosphazenes are known, Dell et al, J. Chem. Soc. 4070 (1965). High molecular weight poly(phenoxyphosphazene) has been described; Kugel et al Inorganic Chem. 5, 1709 (1966).
Phosphonitrilic halides can be prepared by reacting NH.sub.4 Cl with PCL.sub.5 ; Emsley et al, J. Chem. Soc. (A), 768 (1971). Other references dealing with this preparation are cited therein and mentioned below.
Various fire retardants have been suggested for polyesters; confer for example, U.S. Pat. Nos. 2,909,501, 3,285,995, 3,309,425, 3,434,981, 3,794,617, and Canadian Pat. No. 924,731. Hexaphenoxyphosphazene had been suggested for fire retarding polyester fibers.